1. Field
The present disclosure relates generally to a method and a system for improving image quality of edge pixels, when separating an image signal into a set of image planes.
2. Description of Related Art
Scanning and exporting color images to a network has started to become one of the standard features offered by digital multifunction devices. File size of a color image is an important factor while exporting color images. In addition to offering different resolutions, different compression schemes are being offered to reduce the file size of the color image that needs to be exported. One of the popular compression/file formats that are currently being offered is. Mixed or Multiple Raster Content (MRC) representation.
The MRC representation of documents is versatile. It provides the ability to represent color images and either color or monochrome text. The MRC representation enables the use of multiple “planes” for the purpose of representing the content of documents. The MRC representation is becoming increasingly important in the marketplace. It has been already established as the main color-fax standard. It is also offered as a selection in the Scan-to-Export feature, for example, in digital multifunction devices.
In a MRC representation, an image is represented by more than one image plane. The main advantage of the MRC representation of documents is to provide an efficient way to store, transmit, and manipulate large digital color documents. The method exploits the properties of the human vision system, where the ability to distinguish small color variations is greatly reduced in the presence of high-contrast edges. The edge information is normally separated from the smoothly varying color information, and encoded (possibly at higher resolution than 1 bit per pixel) in one of the planes, called the Selector plane. The selector plane may have only one bit per pixel that controls the selection from either foreground or background. Following a careful separation, the various planes could be independently compressed using standard compression schemes (such as JPEG and G4) with good compression and high quality at the same time.
In digital image processing, an edge within an image is referred to a sharp change in local intensity or lightness. In other words, edges are features within an image that possess strong intensity contrast. Edges occur between distinct objects in a scene, or within textures and structure within an object. For instance, typographic characters on a white page background produce distinct edges. Edge pixels in a digital image are those pixels that occur at and about an edge in the image.
FIGS. 13 and 14 show a digital image (e.g., a bar chart) and an edge profile that is created from the digital image, respectively. As shown in FIG. 14, the black areas in the edge profile represent the edges within the digital image. The edge pixels are the pixels that occur at and about the edges (e.g., the dark lines are intended to represent strong edges while the broken lines represent edges with lesser intensity in the edge profile as shown in FIG. 14) in the image.
A three layer segmentor used for MRC representation pushes the edges in foreground and background planes to minimum and maximum values of the current pixel, respectively. In other words, the edge pixels in the foreground, and the background planes are switched to the minimum value or the maximum value, respectively, which adds contrast to the image and provides some background suppression. The minimum and maximum values of each pixel are evaluated based on the local neighborhood of the current pixel. These minimum and maximum values are sometimes very different from the current pixel value, which may cause ringing artifacts around edges and make them appear artificial, resulting in poor image quality.
U.S. Pat. No. 7,242,802, which is incorporated by reference herein in its entirety, describes a prior approach for separating an image signal into a set of image signals. The invention of U.S. Pat. No. 7,242,802 discloses that the output of the foreground values will be correspondingly weighted between the current input signal value and the minimum value of the dependent min-max module based on the amount of segmentation enhancement. Similarly, the output of the background values will vary between the current input signal value and the maximum value of the dependent min-max module in proportion to the amount of segmentation enhancement.
Thus, there is a need for a method and a system for efficiently separating an image into a set of planes, such that the advantages of the MRC representation can be fully exploited. The present disclosure proposes a method and a system to conditionally switch foreground and background edge pixels to either minimum value or maximum value, respectively, or to the current pixel values.